Mundo's experience is similar to many other emergency dispatchers, the faceless voices of calm and reason who help people through their most difficult moments.

These literal first responders go unrecognized for the life-saving work they do, and the emotional and psychological trauma also often goes unrecognized. It is work that can exact a significant toll on dispatchers, sometimes to the point of being diagnosed with post-traumatic stress disorder.

“For a person calling 911, it is the one worst moment of their lives,” former Vancouver 911 dispatcher Rae-Lynne Dicks told ABC News. “They don’t know or understand how the system works other than that’s the number I’m supposed to call when my life is falling apart or something is on fire or somebody’s having a heart attack or any of the many thousands of reasons a person would have to call 911.”

“For the 911 operator, the call-takers, that’s what they hear all day,” Dicks said.

The spectrum of calls includes pocket-dials, stolen bikes, medical emergencies, violence and unimaginable horrors that some call-takers “wish to God” they could forget.

“People think, ‘Oh, they’re just dispatchers. It’s their job. They’re used to it,’” Georgia 911 supervisor Elaina Fincher told ABC News. “No, we’re really not. You don’t ever know what you’re going to get when you pick up that phone. You never know.”

Some emergency centers have both 911 operators, also called call-takers, and dispatchers. Operators answer the calls and talk to members of the public, finding out what their emergency is and where they are. Dispatchers communicate with and deploy police, firefighters and EMS to the scene. In some places, one person does both jobs.

For 911 emergency staffers across the U.S. and Canada interviewed by ABC News, the first thing they must learn to do is to maintain their cool, no matter what is happening at the other end of the phone line.

“The minute you hear somebody scream, your initial reaction is, ‘Oh my God’ and you start freaking out. But you just have to — no matter how hard your heart is pounding — you have to keep your voice calm and it takes a while to learn that,” said a Colorado dispatcher who asked that her name not be used.

While maintaining a calm voice can be learned, many said they just can’t learn to shake some calls. A number of the operators said that calls involving children are particularly difficult as well as calls where they hear someone die, whether it is from injuries or suicide.

Emergency Dispatchers Can Suffer PTSD

“Having that thought in your mind stuck with you that when somebody does decide to pull the trigger while they’re on the phone with you, you’re the last voice that person has heard forever,” Fincher said. “They’ll never hear another voice and yours was the last one.”

“And then hearing somebody admit on the phone that, ‘Hey, I just stabbed my husband in the chest and he’s bleeding’ and stuff like that,” she continued. “Having a person call and say their 5-year-old child’s leg was just amputated by a bear trap … stuff like that just sticks with you.”

Fincher said those are the unforgettable calls that “haunt you, that you wish to God you could just block out.”

Once emergency services are on-scene, the callers usually hang up and the call-takers never know the outcome of the situations.

“After that heightened call, when we hang up, we don’t have any closure. We don’t know the end result of what happened,” said a dispatcher named Victoria from Massachusetts, who asked that her last name not be used. “Imagine yourself reading a really intense novel that you cannot put down and you turn the page and the last two chapters have been ripped out.”

The tolls of the job affect operators’ personal lives in various ways.

Some like Robert Schumacher, a dispatcher in Antioch, Ill., say they work hard to “compartmentalize” the different parts of their lives.

“I try to leave it at the door,” Schumacher said. "One recent call, it came home with me and bothered me for a day or so, but then you have to let it go. There’s nothing I can do personally. I did everything I could to help this person. I’ve never had any issues with PTSD or anything like that.”

Others can’t prevent the traumas of work from seeping into their personal lives.

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They’ll never hear another voice and yours was the last one.

Mundo, the Florida dispatcher, said 911 dispatchers and operators are expected to be as emotionally tough as hardened cops.

“In law enforcement, there’s this sub-culture or subconscious thought that says, if you need counseling, if you need help, then maybe you can’t take it,” Mundo said. “It’s kind of like a hero complex where you feel like you’re saving people all the time and even if you need help, you just want to brush it off and say, ‘You know what, forget it. I’ll just keep going.’ You just want to push through it when you really do need some help.”

Fincher reached her “breaking point” in 2012 when she was at the park with her daughter and realized that she was agitated all the time.

“If we keep this bottled up, we’re bound to explode and have a mental breakdown,” Fincher said. “I had reached a point where I was just so tired all the time, I was angry all the time and I really didn’t know why.”

She went to see a doctor and was diagnosed with PTSD from her job. Part of her treatment included making a Facebook page for 911 operators and dispatchers that provides a community for support and sharing.

The operators and dispatchers celebrate the joys of successful calls, whether it’s keeping a parent calm until they discover that their missing child has been hiding in the closet or talking someone through an injury until help arrives.

Michelle Lilly, a member of the Department of Psychology at Northern Illinois University, co-authored a study on the emotional stress endured by 911 operators and dispatchers.

911 Operators' Long Term Toll

“They’re handling tons of calls in which they are experiencing pretty strong emotional distress and response to them. People handling calls where parents have found their child drowned in a pool and having a sibling in the house with a knife that’s trying to attack the other or having to talk to people who are literally dying within a natural disaster. I mean just horrifying calls," Lily said.

"You don’t have to be on the scene, you don’t have to be a police officer or a firefighter to be traumatized by these calls. So there certainly was PTSD symptomology,” she said.

The National Emergency Number Association said the field has begun to appreciate "the long lasting and severe physical and psychological effects" of the 911 jobs, according to Ty Wooten, NENA's director of Education and Operational Issues.

The group recommends that 911 centers create an eight hour course for employees on recognizing and handling the effects of stress.

Lily said the mental toughness demonstrated by the 911 crews was remarkable and was an illustration of how "resilient" they are.